This invention relates to circuits for measuring rectified a-c voltages in general and more particularly to an improved circuit which gives superior compensation at low voltage values.
When measuring a-c voltages it is customary to first rectify the voltage. When using semiconductor rectifiers measuring errors occur particularly near the zero point because of the threshold values or diode drops in the semiconductor used. Circuits for compensating these errors have been developed. Typically in such circuits additional diodes are connected in series and with a polarity opposite that of the diode of the rectifier. These additional diodes or compensating diodes are connected to an auxilliary voltage source and carry a current which is made to correspond as accurately as possible to the current in the rectifier elements of the rectifier. In addition to requiring a separate auxiliary voltage source circuits of this nature also require that current flowing through the compensation diodes must correspond to the current in the rectifier diodes as closely as possible. Such can be assured only through relatively difficult adjustment and balancing problems constituting a further disadvantage. Furthermore, compensation of the diode threshold measuring error is not possible over the entire measuring range using such circuits. A null error, for example cannot be avoided using the constant auxiliary voltage sources.
Thus, there is a need for an improved circuit of this nature in which measurement errors due to diode threshold values are eliminated to the greatest extent possible throughout all measuring ranges.